This invention relates to electrochemical cells and batteries, and more particularly, to lithium ion cells and batteries.
Lithium batteries are prepared from one or more lithium electrochemical cells. Such cells have included an anode (negative electrode), a cathode (positive electrode), and an electrolyte interposed between electrically insulated, spaced apart positive and negative electrodes. The electrolyte typically comprises a salt of lithium dissolved in one or more solvents, typically nonaqueous (aprotic) organic solvents. By convention, during discharge of the cell, the negative electrode of the cell is defined as the anode. During use of the cell, lithium ions (Li+) are transferred to the negative electrode on charging. During discharge, lithium ions (Li+) are transferred from the negative electrode (anode) to the positive electrode (cathode). Upon subsequent charge and discharge, the lithium ions (Li+) are transported between the electrodes. Cells having metallic lithium anode and metal chalcogenide cathode are charged in an initial condition. During discharge, lithium ions from the metallic anode pass through the liquid electrolyte to the electrochemically active material of the cathode whereupon electrical energy is released. During charging, the flow of lithium ions is reversed and they are transferred from the positive electrode active material through the ion conducting electrolyte and then back to the lithium negative electrode.
The lithium metal anode has been replaced with a carbon anode, that is, a carbonaceous material, such as non-graphitic amorphous coke, graphitic carbon, or graphites, which are intercalation compounds. This presents a relatively advantageous and safer approach to rechargeable lithium as it replaces lithium metal with a material capable of reversibly intercalating lithium ions, thereby providing the so-called xe2x80x9crocking chairxe2x80x9d battery in which lithium ions xe2x80x9crockxe2x80x9d between the intercalation electrodes during the charging/discharging/recharging cycles. Such lithium metal free cells may thus be viewed as comprising two lithium ion intercalating (absorbing) electrode xe2x80x9cspongesxe2x80x9d separated by a lithium ion conducting electrolyte usually comprising a lithium salt dissolved in nonaqueous solvent or a mixture of such solvents. Numerous such electrolytes, salts, and solvents are known in the art.
In the manufacturing of a battery or a cell utilizing a lithium-containing electrode, there is an attempt to eliminate as many undesirable impurities and unstable precursor components as possible. Such undesirable impurities and precursors adversely affect cell performance.
In a lithium battery or cell, it is important to eliminate as many impurities and some precursor components which may affect cell performance. Such impurities and precursor components cause side reactions and are subject to breakdown because they are not electrochemically stable. Loss of performance due to impurities and breakdown of precursor compounds causing undesired side reactions has led to the formation of cell components and assembly of the cell under very controlled conditions. Performance problems have also led to the removal and extraction of as many impurities and precursor components as possible in order to minimize problems. However, extraction techniques for removing such undesired compounds are very time-consuming and very costly. Therefore, what is needed is an understanding of the mechanism causing undesired loss of performance and the resolution of same, which avoids the need for costly and time-consuming process steps; and a new method for forming battery components which avoids the need for costly extraction and purification steps.
The present invention provides a novel composition from which electrochemical cell component films are fabricated which avoids undesired electrochemical breakdown of cell components; and which avoids the need for complex purification steps to reduce or substantially eliminate precursor components subject to electrochemical breakdown.
The components of the cell are formed from a specifically selected class of new plasticizers which are resistant to decomposition by electrochemical breakdown. The new class of plasticizers are characterized by electrochemical stability at least up to about 4.5 volts.
In addition to their electrochemical stability, the plasticizers of the invention have properties similar to those desired in an electrolyte solvent.
The plasticizers of the invention are generally characterized as dibasic esters based on adipates. They have the general formula as shown in Table I, where xe2x80x9cRxe2x80x9d represents a low alkyl selected from methyl, ethyl, propyl, butyl, pentyl and hexyl. Accordingly, xe2x80x9cRxe2x80x9d represents a low alkyl, having up to six carbon atoms. The plasticizers of the invention are further characterized by electrochemical stability up to about 4.5 volts, and by disassociatingly solubilizing the metal salt of the electrolyte. The plasticizers of the invention have characteristics consistent with desired electrolyte solvents, and they may be used as all or part of the solvent mixture. However, it is preferred to remove at least a portion of the plasticizer after casting the film.
The characteristics of the plasticizer include the ability to disassociatingly solubilize the metal salt used for ion transport in an electrochemical cell. Advantageously, the plasticizer need not be extracted completely from precursor components, the electrode and/or electrolyte, before final assembly of the cell. This is because the plasticizer and the solubilized salt become distributed within the separator of the completed cell where the plasticizer along with other components of the solvent mixture are dispersed for ion transport. Preferably, the solvent mixture comprises, besides the plasticizer, at least one of those solvents selected from the group consisting of ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), dibutyl carbonate (DBC), diethoxy ethane (DEE), ethyl methyl carbonate (EMC), butylene carbonate (BC), vinylene carbonate (VC), propylene carbonate (PC), and mixtures thereof. Since the plasticizer is not a preferred solvent, it preferably constitutes a relatively small portion of the solvent mixture. The plasticizer is preferably present in an amount less than the amount by weight of any other one of the solvents included in the mixture. Advantageously, the plasticizer is miscible with the aforesaid common solvents. Other characteristics of the dibasic esters of the invention based on adipate include, based on the exemplary dimethyl adipate (DMA), a boiling point of 109-110xc2x0 C.; a melting point of about 8xc2x0 C.; vapor pressure of about 0.2 mm; specific gravity of about 1.063; and purity on the order of 98-99%. The plasticizer in appearance is a colorless liquid, dialkyl adipate.
Although the plasticizer of the invention may remain as a part of the cell component (electrode and/or separator) after its fabrication, it is preferred to remove at least a portion of the plasticizer. In any event, the solubilizing plasticizer of the invention, forming a part of the solvent mixture, is present in an amount not greater than the amount by weight of any other one of the organic solvent components. The preferred plasticizers are dimethyl adipate (DMA) and diethyl adipate (DEA). The characteristics of dimethyl adipate (DMA) as outlined above are shown in Table II. The preferred dimethyl adipate is shown as an entry in chemical structural formula in Table I.
The electrochemical cell of the invention comprises a first electrode; a counter-electrode which forms an electrochemical couple with the first electrode; and an electrolyte. The electrolyte comprises the solute in solvent mixture. The solute is essentially a salt of the metal. In the case of a lithium ion battery, this is a lithium salt, such as LIPF6. According to the invention, at least one of the electrodes comprises an active material; a polymeric material functioning as a binder; and a plasticizer for the polymeric material, where the plasticizer is at least one compound selected from the group of dibasic esters derived from adipate, according to the invention. Preferably, in the case of a metal oxide electrode, the electrode composition further comprises a conductive diluent such as graphite. The preferred polymeric binder material is preferably a co-polymer of polyvinylene difluoride (PVDF) and hexafluoropropylene (HFP). In another aspect, the electrolyte/separator film is formed from the co-polymer and plasticizer.
The plasticizers of the invention solve the difficult processing problems associated with removal of conventional plasticizers after formation of cell components and before their assembly into a cell. The plasticizer of the invention may be used to formulate any of the polymeric components of the cell, positive electrode, negative electrode, and electrolyte/separator. Plasticizers of the invention comprising adipate derivatives, esters, are highly desirable due to their stability. Plasticizers of the invention are stable under atmospheric conditions on exposure to oxygen, humidity, and importantly, are electrochemically stable. This is in contrast to plasticizers conventionally used to form cell components. Such conventional plasticizers must be removed prior to assembly of the cell as they are not electrochemically stable. An additional advantage is that the plasticizer of the invention has characteristics consistent with properties desired for a solvent and functions as a part of the solvent mixture when included in an electrochemical cell. Therefore, advantageously, the plasticizers of the invention become part of the electrode formulation performed characteristic function as a plasticizer during formation of cell components from precursor compounds, and then they remain as a part of the cell component when the cell is assembled.
Objects, features, and advantages of the invention include an improved electrochemical cell or battery having improved charging and discharging characteristics; which maintains its integrity over a prolonged life cycle as compared to presently used batteries and cells. Another object is to provide electrode mixtures comprising constituents which are stable when cycled in an electrochemical cell, and which demonstrates high performance, and which does not readily decompose during cell operation. It is also an object of the present invention to provide cells which can be manufactured more economically and conveniently, and to provide cells with electrode and electrolyte components which are compatible with one another, avoiding problems with undesired reactivity, breakdown, and degradation of cell performance.
These, and other objects, features, and advantages, will become apparent from the following description of the preferred embodiments, claims, and accompanying drawings.